1. Field of the Invention
The present invention relates to a method for controlling wireless access, and more particularly, to a method for performing handover in a broadband wireless access system. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for a mobile subscriber station effectively maintaining a sleep mode in a broadband wireless access system mode after handover.
2. Discussion of the Related Art
A broadband wireless access system supports sleep mode in a mobile subscriber station (MSS) in order to minimize power usage in a battery. Sleep mode includes a sleep interval in which the MSS does not transmit with a base station (BS) in order to save power, and a listening interval which takes place between two sleep intervals to determine whether downlink traffic is transmitted to the MSS from the BS.
The broadband wireless access system includes three classes or categories of sleep mode based on the features of currently selected traffic. Sleep mode can also be expressed as power saving mode. There are three types of power saving class modes, further defined as power saving class parameters. A power saving class parameter 1 (“Class 1”) includes an initial sleep window, final window base, final window exponent, listening window, and start frame number of sleep window parameters. Moreover, the objective of Class 1 type is to incorporate features such as Best Effort (BE) which includes features from the existing internet traffic or Non-Real-Time Variable Rate (NRT-VR) with varying transmission rate.
A power saving class parameter 2 (“Class 2”) seeks to incorporate a feature of Real-Time Variable Rate (RT-VR) which varies Voice over Internet Protocol (VoIP) or transmission rate. Class 2 includes an initial sleep window, listening window, and start frame number of sleep window parameters.
A power saving class parameter 3 (“Class 3”) includes periodic transmission of a control message to the MSS in sleep mode along with Downlink Channel Description/Uplink Channel Description (DCD/UCD) and a MOB-NBR-ADV messages for receiving information of neighboring BS coverage areas for handover or transmitting data for multicast transmission. Class 3 includes a final window base, final window exponent, and start frame number of sleep window parameters.
With respect to Class 1, the operation of sleep mode of the MSS is affected by the operation of the listening interval and the sleep interval which can be increased at a specified rate. The duration of the listening interval is determined by a MOB-SLP-REQ message and a MOB-SLP-RSP message. The MOB-SLP-REQ message is transmitted from the MSS requesting a base station (BS) if the MSS can enter sleep mode. The MOB-SLP-RSP message is received by the MSS from the BS in which the BS transmits a response to the request. During the listening interval, the MSS receives traffic indication message (MOB-TRF-IND) which is a control message of Medium Access Control (MAC). From the message, the MSS acquires information on whether any downlink traffic is transmitted or whether ranging procedure for maintaining a downlink coding type associated with uplink transmission maintenance and signal quality has to be performed.
The MSS only receives minimum amount of downlink signals from the BS in order to save power during the sleep interval. Even during sleep mode, the MSS can scan neighboring base stations in case handover has to be performed and can perform ranging procedure.
The MSS in sleep mode receives the MOB-TRF-IND message during the listening interval and determines whether to continue sleep mode, terminate sleep mode to receive downlink signal, or perform ranging procedure during sleep interval.
FIG. 1 illustrates the operation of class 1 sleep mode. In FIG. 1, the MSS transmits the MOB-SLP-REQ message is transmitted to the BS to request for entry to sleep mode (S11). The MOB-SLP-REQ message includes information on an initial sleep interval, final sleep interval, and listening interval. The information related to the initial sleep interval, final sleep interval, and listening interval can be expressed in frames.
If the BS grants the MSS to enter sleep mode, the MOB-SLP-RSP message is received by the MSS (S12). The MOB-SLP-RSP message includes information on the initial sleep interval, final sleep interval, listening interval, and sleep mode entering point. The information of the initial sleep interval, final sleep interval, and listening interval included in the MOB-SLP-RSP message can be same or different from the information contained in the initial sleep interval, final sleep interval, and listening interval of the MOB-SLP-REQ message.
After receiving the MOB-SLP-RSP message, the MSS enters and remains in sleep mode for a specified duration. Upon expiration of the initial sleep interval, the MSS enters the listening interval. The MSS receives the MOB-TRF-IND message during the listening interval (S13) and receives any downlink traffic if transmitted from the BS.
Absent transmission of downlink traffic, the MSS enters subsequent sleep interval and remains in sleep mode for a specified duration. The duration of this second or subsequent sleep interval is measured from the duration of the first sleep interval. For example, the duration of the second sleep interval can be doubled from the duration of the first sleep interval. Subsequently, the duration of the third sleep interval is measured based on the size of the second sleep interval.
After the second sleep interval expires, the MSS enters another listening interval. The MSS receives the MOB-TRF-IND message during the listening interval (S14) and determines whether any downlink traffic exists. If there is downlink traffic, the MSS terminates sleep mode and returns to normal mode to receive downlink traffic from the base station.
However, if the MSS does not detect any downlink traffic during the listening interval, the duration of the subsequent sleep intervals are increased in the same manner as described above. Such a progressive increase in the duration of the sleep intervals is repeated until the duration of the sleep interval matches the duration of the final sleep interval. At this point, the duration the sleep interval is maintained at equal duration of the final sleep interval.
The information on the final sleep interval included in the MOB-SLP-REQ/RSP messages can be expressed by the final sleep window base and final window exponent. Here, the final sleep interval can be calculated using Equation 1.Final Sleep Window=Final Sleep Window Base*2Final Window Exponent  [Equation 1]
If the MSS has to perform handover from sleep mode, for example, the MSS enters a neighboring BS coverage area, terminates sleep mode, and performs handover.
As discussed above with respect to increase in the duration of sleep intervals, the subsequent duration of the sleep interval is longer than the duration of the previous sleep interval for the MSS in sleep mode. The reason for this is because the BS determines that if no downlink traffic was transmitted during the current sleep interval, there is high probability that downlink traffic will not take place in the next sleep interval. Therefore, the BS increases the durations of the sleep interval after each listening intervals until the duration of the sleep interval matches the duration of the final sleep interval.
However, if the MSS terminates sleep mode to perform handover, the MSS returns to sleep mode after handover is completed. In such event, the MSS has to repeat the process of building up the duration of the sleep intervals. In other words, the MSS has to send the MOB-SLP-REQ message to request to enter into sleep mode to the BS, as did in the beginning of the process. Consequently, the MSS has to re-enter the sleep mode by making a request to the BS and progressively build up the duration of the sleep interval starting with the initial sleep interval.
Generally, handover usually takes place as a result of the MSS being in motion, regardless whether transmission of downlink traffic takes place. Therefore, it is inefficient for the MSS to re-enter sleep mode from the beginning or from the initial sleep interval which has reached a certain sleep interval or even the final sleep interval just because handover took place. Furthermore, because the duration of the sleep interval re-enters from the beginning, the listening intervals appear more frequently and unnecessarily transmits the MOB-TRF-IND messages accordingly.
For example, if the MSS having 1024 frames in the sleep interval executes handover, in order to reach sleep mode having 1024 frames in the sleep interval in the new cell, the MSS has receive nine MOB-TRF-IND messages assuming the initial sleep interval includes two frames. As a result, resources and power are wasted to transmit and receive nine MOB-TRF-IND messages.
With respect to Class 2, sleep mode of the MSS takes place in accordance with the listening interval and the sleep interval having a fixed duration. FIG. 2 illustrates the operation of Class 2 sleep mode. In FIG. 2, the MSS transmits the MOB-SLP-REQ message to the BS to request for entry to sleep mode (S21). The MOB-SLP-REQ message includes information on an initial sleep interval and listening interval. The information related to the initial sleep interval, final sleep interval, and listening interval can be expressed in frames.
If the BS grants the MSS to enter sleep mode, the MOB-SLP-RSP message is received by the MSS (S22). The MOB-SLP-RSP message includes information on the initial sleep interval, listening interval, and sleep mode entering point. The information of the initial sleep interval, final sleep interval, and listening interval included in the MOB-SLP-RSP message can be same or different from the information contained in the initial sleep interval and listening interval of the MOB-SLP-REQ message.
After receiving the MOB-SLP-RSP message, the MSS enters and remains in the initial sleep interval for a specified duration. Upon expiration of the initial sleep interval, the MSS enters the listening interval. During the listening interval, the MSS acquires a corresponding period in order to maintain uplink/downlink traffic with the BS and can receive downlink traffic transmitted from the BS (S23).
After the expiration of the listening interval, the MSS re-enters sleep mode for the duration of the initial sleep interval. Again, the MSS returns to the listening interval after expiration of the sleep interval at which point, the MSS can receive downlink traffic. If the MSS desires to terminate sleep mode, the MOB-SLP-REQ can be used during the listening interval to request for termination of sleep mode (S24) and transmits user data to the BS.
However, while in sleep mode, if the MSS returns to normal mode after terminating sleep mode to perform handover, the MSS re-enters sleep mode after handover is completed. In such event, the MSS has to transmit the MOB-SLP-REQ message to the BS as was done previously to enter sleep mode and receive the MOB-SLP-RSP message. As a result, the MSS cannot re-enter sleep mode immediately after completing handover operation but has to repeat the procedure of entering sleep mode.
With respect to Class 3, the MSS enters sleep mode based on the information transmitted via a periodic ranging procedure. FIG. 3 illustrates the operation of Class 3 sleep mode. In FIG. 3, the MSS acquires uplink transmission parameter by performing the ranging procedure periodically in order to maintain uplink transmission with the BS. In other words, the MSS transmits a ranging request (RNG-REQ) message (S31) and receives a ranging response (RNG-RSP) message from the BS (S32).
The RNG-RSP message includes information on uplink transmission parameter and power saving class parameter, which includes the final sleep interval and sleep mode entry point. The MSS enters sleep mode after receiving the RNG-RSP message and remains in sleep mode for the duration of the final sleep interval. After the sleep interval expires, the MSS communicates with the BS in normal mode and periodically performs handover procedure (S33, S34). During the ranging procedure, the MSS receives the RNG-RSP message (S34), and if the power saving parameter is included in the RNG-RSP message, the MSS enters sleep mode according to the RNG-RSP message.
However, if the MSS terminates sleep mode and returns to normal mode to perform handover operation, the MSS re-enters sleep mode after performing handover operation. In this case, the MSS has to transmit the same RNG-REQ message or the sleep request message as the one transmitted prior to performing handover. As a result, the MSS has to receive the sleep request message or the RNG-RSP message before allowed to re-enter sleep mode.